Cathode-ray tube system and method of operation



June 25, 1946., H. w. I EvERENz 2,402,75

CATHODE RAY\TUBE SYSTEM AND'METHOD 0F OPERATION,

Fied .July 23, 1942 "hnnynunnnnunnunnunu,

22:0 4Roo sbo PatentedJune 25, 1946 CATHODE-RAY TUBE SYSTEM AND METHOD F OPERATION Humboldt W. Leverenz, South Orange, N. J., assignor to Radio Corporation of America, 'a corporation of Delaware Application July 23, 1942, Serial No. 451,984

1 Claim. (Cl..1787.5)

My invention relates to cathode ray tubes, methods of operation, and systems, and particularly to tubes and systems incorporating targets of the luminescent material type.

It is known that alkali halides such as potassium chloride have the property of coloring and developing color centers to form a dark trace under electron bombardment. Thus, when such an alkali halide target is scanned by an electron beam, electrons are injected into the crystal lattices, thereby modifying the light absorbing properties of the halide so that when illuminated, a dark trace having relatively good contrast with respect to a uniform light background from an auxiliary light source is obtained. The recent development of aircraft position and indicating equipment utilizing cathode ray tubes wherein the electron beam of the tube is sequentially pulsed to form on the target a, dark trace representative of the position or trajectory of the aircraft necessitates the development of high contrast between the areas of the target indicating aircraft position and distances with respect to the surrounding areas of the target. While the halide type of target has certain advantages over heretofore known luminescent targets such as de-l scribed in my copending application Serial No. 383,893, filed March 18, 1941, notably in reduction of flash ratio, greater contrast, and in addition a tube having greater life, it is necessary to provide better operation and to avoid early tube replacement.

Inasmuch as received signals representative of the air craft position and distance aretransient, it is necessary to portray the information of which these signals are representative over a relatively long period of time following their reception. For example, the persistance of the portrayed intelligence must be of the order of two seconds to one minute. The conventional halide targets do not give entirely satisfactory contrast over this desired period. Furthermore, the ordinary halide targets have an inherent disadvantage in that repeated bombardment or very strong bombardment by cathode rays produces a permanent discoloration after a very short period of use. In addition. such halides decompose liberating halogen ions which destroy the cathode. I

It is an object of my invention to provide a more eicient dark trace tube and system for portraying transient phenomena over a relatively long period of time. Another object is to provide a tube and system having better life and greater ycontrast than obtainable in tubes and systems used heretofore. Another object is to provide an improved tube and system operable without anauxiliary light source. A further object is to provide a method of developing dark traces of transient phenomena wherein the phenomena may be observed over a relatively long period of time; and it is a still. further object to provide a tube and system having high contrast and a low ash ratio. l

These, and other objects, features, and advantages of my invention will be apparent from the following description and the accompanying drawing wherein,

Fig. 1 shows a cathode ray tube incorporating a target structure and system made and operated in accordance with my invention;

Fig. 2 shows a curve useful in explaining my invention; and, I

Fig. 3 shows a portion of another suitablev target structure for use in the tube of Fig. 1.

In accordance with my invention, I'excite a luminescent screen to uniform high brilliancy by one means or by a group of means, such as cathode'rays, ultra violet, or X-ray radiation and then de-excite pattern areas of the brilliantly violet or other exciting energy. Further, in accordance with my invention, I maintain the intensity of such excitation near the vsaturation level of. the phosphor and I then scan the phosphor screen with a high intensity electron beam of suiicient density to exceed the current saturation level of the phosphor to an extent sufcient to momentarily burn the screen. Therecovery of the phosphor screen material to its original efficiency may be controlled by varyingl the nature of the phosphor, the particle size of the phospho'i-thermal insulation of the phosphor particles from each other and from thetube wall. screen thickness, heat capacity of the screen, and temperature of the screen.

A tube and system incorporating my invention is shown in Fig. 1, the tube comprising an evacuated envelope I preferably having -two arm or neck sections 2 and 3 oppositely disposed from a phosphor screen 4 of material or materials referred to hereinafter. my invention wherein the screen 4 is initially excited to uniform high brilliancy by cathode rays, the neck portion 2 may incorporate an electron gun either of the the focusing or non-focusing conventional types. The neck section 3 is provided with an electron gun 6 to develop and project a focused electron beam upon the screen 4 immediately following or during excitation to high brilliancy. Each of the electron guns 5 and 6 are supplied with conventional operating voltages as shown, provision being made to signal modulate the electron gun 6 as by conventional grid control. Such modulation may be effected by'elect'ronic means whereby upon reception of signal a high beam current' is obtained whereas for no signal the gun operates substantially at cut-off. I have shown in Fig. 1 deection means l for scanning the electron beam from the gun 5 over the screen 4 although it is not necessary to use such scanning means when the gun 5 is designed to produce an unfocused electron beam provided the entire effective area of the screen 4 is subjected to electron bombardment near the current saturation level of the screen material. Further scanning means 8 however are necessary to deflect the focused electron beam from the gun 6 over the effective areas of the screen 4 to produce the dark trace when the areas are subjected to excitation from the gun 5 beyond the current saturation level of the phosphor. In operation the electron gun 5 either sprays a defocused wash of electrons on the screen 4 to keep the luminescence output of the screen near the current saturation level or else rapidly scans the screen to obtain the same effect when considered in conjunction with the persistence of the human eye. The second electron gun 6 operates independently of the rst with respect to scanning speed, pattern, modulation, voltage, and spot size and burns the transitory dark trace on the bright background. Consequently, by modulating the current intensity of the electron beam from the gun 6, such as by reducing the electron gun 6 grid bias during reception of signals, the dark trace representative of the signals is produced, provided the current intensity is suicient to exceed the current saturation level of the phosphor screen material and decrease the luminescence of the bombarded screen element. The decrease in luminescence is probably largely due to raising the temperature of the phosphor particles above the temperature break-point, although the luminescence may be simultaneously reduced by a reduction in secondary emission efliciency of the phosphor.

The decrease in luminescence may be explained by reference to Fig. 2 which is a curve representing the decrease in luminescence with increase in temperature, that is, increase in electron beam intensity. While effects other than an increase in temperature accentuate loss of luminescence, this curve shows that for a representative phosphor material, such as zinc sulphide, the luminescence decreases rapidly upon exceeding a critical temperature termed the luminescence break-point. Therefore in operation the screen is excited to such a degree and the temperature is such thatprior to de-excitation by the high intensity electron beam the visible luminescence output is as represented to the left o f the breakpoint whereas after de-excitation the lumines- In the modications of cence is to the right of the break-point. The term current saturation level" is thus equivalent to the term "luminescence break-point" and the term equivalent current saturation level is similarly equivalent to this break-point whether electron, ultra violet or other excitation means with or without auxiliary heating of the screen is utilized to initially excite the screen to a high degree of luminescence prior to de-excitation to form the dark trace.

Various phosphor materials may be utilized for the screen 4 although it is essential that the luminescent material have good recovering power following formation of the dark trace. Oxygen-containing phosphors, such as manganeseactivated zinc silicate or other silicates, germanates, aluminates, and gallates are generally preferable to sulphur-containing phosphors. However, such sulphides as zinc sulphide, silveractivated zinc sulphide, copper-activated zinc cadmium sulphides, as well as seleno-sulphides may be used to advantage for certain applications, especially at low voltages where sulphides, afford greater luminescence eiliciency.

As indicated above, the excitation of the screen 4 to uniform high brilliancy may be by means other than cathode ray excitation such as by ultraviolet, ionic-bombardment or X-ray excitation. For example, by making the tube face or foundation on which the screen 4 is deposited of ultraviolet transmitting material such as Vycor glass, quartz or Corning 974 HW glass the screen may be excited to a high level of brilliancy from an external ultraviolet source predominant in Wave lengths of 2537 A. or 3650 A., depending upon the type of phosphor used, either in addition to or in place of excitation from the electron gun 5. If ultraA violet or X-ray excitation alone be employed, an external or internal means may be provided to raise the temperature of the screen to the break-point shown in Fig. 2.

It must be appreciated that it is essential to excite the phosphor screen substantially to the` saturation level either by cathode ray or other excitation means prior to cathode ray bombardment i beyond the current saturation level for dark trace formation because the screen must be' o n the threshold of burning prior to producing the transient burning effect by further electron bombard- 1 ment. This bombardment effectively reduces the emciency of the screen so that there is an actual loss of luminescence to form the dark trace. For maximum contrast the initial excitation should be preferably at least oi' the maximum excitation obtainable at current saturation so that the electron beam developing the transient phenomena in the form of the dark trace may produce a trace which is high 1n contrast and of a persistence sufiicient for the intended use such as indicated above.

'I'he phosphor screen 4 may be of the composite type as disclosed in my above mentioned copending application operating on ,the cascade principle. In accordance with this prior teaching a layer of luminescent material having predetermined absorption and emission spectra is provided adjacent a second phosphor layer having an absorption spectrum overlapping the emission spectrum of the first layer. Alternatively, an electron barrier layer may be utilized between the two aforementioned phosphor layers as described in my copending application Serial No. 417,269, led October 3l, 1941.

Referring to Fig. 3, which shows diagrammatically, such a screen corresponding to and replacing the screen 4 of Fig. 1, the rst phosphor layer III comprises material such as silver-activated zinc sulphide or other phosphor having an energy absorption spectrum in the violet or ultraviolet region and an emission spectrum in the blue or near violet region. The second layer. I I comprises phosphor material having an excitation spectrum 'overlapping the emission spectrum of the layer I0. The two layers may be separated by an electron barrier layer I2 as shown, although use of this barrier layer is not essential.

In operation of the structure of Fig. 3 the phos-1 phor layer IU is excited to uniform high brilliancy by ultraviolet light, X-rays, cathode rays, or other corpuscular or radiant energy, the luminescence near the break-point shown in Fig. 2. Following excitation of layer II by -this means, I project a high intensity electron beam having the properties of the beam from the electron gun 6 of Fig. l. upon the layer II and scan-the beam thereover to produce the dark trace. I control the intensity of the beam incident on the layer II maintaining it above the current saturation level of the phosphor material comprising the layer Il so that desired areas of the layer' II are burned to render the dark trace visible on the general luminescent background of the layer II. The barrier layer I2 is provided to prevent the beam incident upon the layer II penetrating within the layer I0, although the velocity of this beam and the thickness of the layer I I may be controlled during manufacture to prevent such penetration without use of the barrier layer I 2.

- As stressed above, and for maximum contrast, it is desirable that the layer I I be excited to within approximately 90% of the equivalent current saturation level under which the layer Il would be initially excited by a cathode ray beam to enable the high intensity beam incident thereon to produce a dark trace of good visibility.

A further modication of my invention com-k` Heating. means prises a, tube such as shown in Fig. 1. except having only one electron gun such as the gun 6 and a single layer screen, such as the screen 4. I operate such Va tube by scanning the screen over a raster area at a rate greater than the persistence of vision such vas in conventional television Kinescope tubes, the raster area being excited at or near maximum luminescence by maintaining a low bias on thev electron gun control grid. Y

The signal to be converted into a dark trace is then superimposed on the control grid in a positive potentialdirection with respect to the electron gun cathode to reduce the grid bias and increase the beam current. The dark trace then` comprises a series of dark dots or short lines along the raster lines and due to persistence of vision and of burning is viewed as a true dark trace.

My method of operation comprises in essence developing luminescence of high intensity approaching the equivalent current saturation of a phosphor and reducing the effective luminescence by exceeding the current saturation level of the phosphor. Furthermore, it will be appreciated that a great number of phosphors may be utilized in performing this method of operation and that I am not limited to the particular apparatus de- .scribed inasmuch as the initial excitation may be by corpuscular or undulatory energy produced in a number of ways and the excitation beyond the current saturation characteristic may be produced electronically or ionically from various types of structures either of the thermionic of photo-ionic type.

I claim: e

A cathode ray'tube and system comprising a tube having two electron guns and a luminescent screen oppositely disposed from said electron guns, means Ato develop a beam of electrons from `one of said guns of sufficient intensity to render said screen highly luminescent, means to develop a beam of electrons from the other of said guns of greater intensity than said rst beam and of an intensity exceeding that necessary to obtain maximum luminescence when incident upon said screen and means to scan said second beam over said screen to develop a dark trace observable by contrast with the high luminescence produced by said rst beam.

HUmoLD'r W. LEvERENz. 

